Frankly, I think there has been a large degree of intransigence on both sides. The free font advocates have refused to identify the fonts that they want to be considered and why they should be considered other than the fact that they are free, and the designers have refused to take any initiative on considering free fonts. The free fonts that I know have been considered are:
* DejaVu Serif. Conclusion: Widely installed, but horribly ugly and looks nothing like the style desired by the designers.
* Nimbus Roman No9 L. Conclusion: Basically a clone of Times. Most Linux systems map Times to Nimbus Roman No9 L, so there is no advantage to specifying "Nimbus Roman No9 L" rather than "Times" (which also maps to fonts on Windows and Mac).
* Linux Libertine. Conclusion: A well-designed free font that matches the look of the Wikipedia wordmark. Unfortunately, it is not installed by default on any systems (as far as anyone knows) but is bundled with LibreOffice as an application font. If MediaWiki were using webfonts, this would likely be the serif font of choice rather than Georgia, but since we are relying on pre-installed fonts, it would be rather pointless to list it.
* Liberation Sans. Conclusion: Essentially a free substitute for Arial. Like Nimbus Roman, there is no advantage to specifying "Liberation Sans" instead of "Arial" (which is at the bottom of the sans-serif stack) since Linux systems will map to Liberation Sans anyway, while other systems will apply Arial.
As to proving the quality of Georgia and Helvetica Neue, I don't think the designers have done that, but I also haven't seen any evidence from the free font advocates concerning the quality of any free fonts. So in my view, both sides of the debate have been delinquent.
Ryan Kaldari